1) I've found the USB2->IDE adaptors works as well (and in some cases better) than direct IDE for CD/DVD drives. This is true only AFTER you boot into windows. During installation of Windows (i.e. Access to the CD/DVD is via BIOS routines), performance is truly awful.
3) Save yourself some grief. Just get the Opus.
4) I'm not sure but during your planning be aware that the Audigy NX USB shares its line-in and mic connections. You can use one or the other, not both. As you said, you can always fall back to the on-board audio if you need an extra set of connections.